The present invention is directed, in general, to wireless networks and, more specifically, to an adaptive digital pre-distortion correction circuit for use in an RF transmitter.
Wireless networks, and cellular telephone networks in particular, have become ubiquitous in society. Reliable predictions indicate that there will be over 300 million cellular telephone customers by the year 2000. In order to maximize the number of subscribers that can be serviced in a single cellular system, frequency reuse is increased by making individual cell sites smaller and using a greater number of cell sites to cover the same geographical area. To maximize usage of the available bandwidth in each cell, a number of multiple access technologies have been implemented to allow more than one subscriber to communicate simultaneously with each base transceiver station (BTS) in a wireless system. These multiple access technologies include time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA). These technologies assign each system subscriber to a specific traffic channel that transmits and receives subscriber voice/data signals via a selected time slot, a selected frequency, a selected unique code, or a combination thereof.
Every cellular base station has a RF transmitter for sending voice and data signals to mobile units (i.e., cell phones, portable computers equipped with cellular modems, and the like) and a receiver for receiving voice and data signals from the mobile units. It is important that the RF power amplifier in a transmitter operate in a highly linear manner, especially when amplifying a signal whose envelope changes in time over a wide range, as in CDMA and multi-carrier systems. It also is important that the RF transmitter operate efficiently under high-power conditions. It also is important that RF amplifiers having good linearity characteristics across a wide range of operating conditions are required because wireless systems cannot tolerate large amounts of signal distortion and may not violate the IS 95 bandwidth requirements regarding spectral spreading effects.
Spurious spectral components are introduced when a signal peak is sufficiently large to saturate an RF amplifier in the transmitter. The RF transmitters in wireless networks in which digital signals have high peak-to-mean ratios, such as CDMA and multi-carrier systems, are frequently xe2x80x9cbacked offxe2x80x9d from full power (or peak power) to avoid clipping the signal peaks. For example, RF power amplifiers in some CDMA systems need more than 10 dB of xe2x80x9coverheadxe2x80x9d space to protect the peak CDMA signal power from clipping. Unfortunately, leaving this much overhead significantly reduces the power efficiency of the RF power amplifier and increases the power consumption and the cooling requirements of the base transceiver station.
A number of techniques are known to try to minimize the amount of overhead an RF power amplifier requires, including feedforward, feedback, and pre-distortion. Each technique has its drawbacks, however. Feedforward systems require a large error power amplifier in the correction loop, which lowers the overall power amplifier efficiency. Feedback systems introduce a delay in the feedback signal, which limits the signal bandwidth to a few MHz. Pre-distortion systems typically exhibit low correction efficiency.
There is therefore a need in the art for improved wireless networks that use more efficient RF power amplifiers. In particular, there is a need for improved RF power amplifiers that can operate more closely to full power in systems having high peak-to-mean digital signal ratios without generating spurious spectral components when a large signal peak is encountered. More particularly, there is a need for improved RF power amplifiers that require less xe2x80x9coverheadxe2x80x9d to prevent sudden large peaks from being clipped due to saturation of the RF power amplifier.
To address the above-discussed deficiencies of the prior art, it is a primary object of the present invention to provide a pre-distortion correction circuit for use in an RF transmitter having a transmit path capable of receiving a digital input baseband signal and generating therefrom a modulated RF output signal. The pre-distortion correction circuit adaptively corrects an amplification distortion caused by an RF power amplifier in the transmit path. In an advantageous embodiment of the present invention, the adaptive pre-distortion circuit comprises 1) input sampling means coupled to an input of the transmit path capable of capturing from the digital input baseband signal a first input sample of amplitude X; 2) demodulation circuitry coupled to an output of the transmit path capable of receiving and demodulating the modulated RF output signal to thereby produce a digital output baseband signal; 3) output sampling means coupled to the demodulation circuitry capable of capturing a first output sample from the digital output baseband signal corresponding to the first input sample; and 4) processing means capable of comparing the first input sample and the first output sample and determining therefrom a pre-distortion correction value corresponding to the amplitude X.
According to an exemplary embodiment of the present invention, the processing means adds the pre-distortion correction value to a subsequently received input sample of amplitude X.
According to another embodiment of the present invention, the processing means comprises a table for storing the pre-distortion correction value.
According to still another embodiment of the present invention, the processing means modifies the pre-distortion correction value in response to a subsequent comparison of a second input sample of amplitude X with a second output sample corresponding to the second input sample.
According to yet another embodiment of the present invention, the processing means is capable of determining if the amplitude X is sufficiently small to ensure that the amplification distortion caused by an RF power amplifier is negligibly small and, in response to the determination, determines a scaling factor for the output samples.
According to a further embodiment of the present invention, the processing means scales the output samples, determines the pre-distortion correction values, and adds the pre-distortion correction values to the look-up table.
According to a still further embodiment of the present invention, the processing means modifies subsequently received input samples of amplitude X according to a value in the look-up table that corresponds to the amplitude X.
According to a yet further embodiment of the present invention, the processing means modifies subsequently received input samples according to a value in the look-up table regardless of the amplitude of the input samples.
The foregoing has outlined rather broadly the features and technical advantages of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features and advantages of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they may readily use the conception and the specific embodiment disclosed as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.
Before undertaking the DETAILED DESCRIPTION, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms xe2x80x9cincludexe2x80x9d and xe2x80x9ccomprise,xe2x80x9d as well as derivatives thereof, mean inclusion without limitation; the term xe2x80x9cor,xe2x80x9d is inclusive, meaning and/or; the phrases xe2x80x9cassociated withxe2x80x9d and xe2x80x9cassociated therewith,xe2x80x9d as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term xe2x80x9ccontrollerxe2x80x9d means any device, system or part thereof that controls at least one operation, such a device may be implemented in hardware, firmware or software, or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior, as well as future uses of such defined words and phrases.